Method and apparatus for treating vascular lesions

ABSTRACT

An apparatus and method is disclosed for treating vascular lesions. In the preferred embodiment, an intracavity, frequency doubled Nd:YAG laser is used to generate output pulses having a duration of 0.5 to 10.0 milliseconds. This laser output is used to irradiate the lesions. The laser energy is absorbed in the blood of the vein, causing it to coagulate and collapse. The long pulse duration helps to minimize bleeding while controlling thermal damage to surrounding tissue.

PRIOR APPLICATIONS

This application is a continuation of Ser. No. 369,465, filed on Jan. 6,1995 now U.S. Pat. No. 5,558,667which is a continuation-in-part of priorapplication Ser. No. 08/355,512, filed Dec. 14, 1994, now abandoned.

TECHNICAL FIELD

The subject invention relates to a method and apparatus for treatingvascular lesions. In the subject invention, the vascular lesions aretreated using a flashlamp pumped, intracavity doubled, solid state lasergenerating output pulses having a duration of 0.5 to 10 milliseconds.

BACKGROUND OF THE INVENTION

There has been significant interest in developing laser systems whichcan be used to treat various forms of vascular lesions. The type ofvascular disorders that have been investigated include port wine stains,face veins, telangiectasis, and birth marks. A wide variety of medicallaser systems have been proposed and introduced to treat these variousdisorders.

The prior art lasers were designed to generate an output wavelengthwhich is absorbed by constituents in the blood. When the vein isirradiated, the blood is heated, causing thermal damage to the vein. Thedamaged vein will thrombose and collapse so that blood will no longerpass through the vein.

The most effective laser systems are designed to deliver a relativelyhigh amount of energy in a short period of time. If the energy isdelivered over too long a period, significant thermal damage will occurin regions beyond the vein being treated. In order to avoid this problemand generate higher powers in a short period of time, most prior artsystems generated a pulsed output.

One common method of generating short, high energy pulses is to use aQ-switch. In a Q-switched laser, the gain medium is excited during aninitial period when lasing does not occur. The Q-switch is then opened,allowing the energy stored in the gain medium to be coupled out of theresonator. Q-switched laser pulses, while having high energy, tend to berelatively short, on the order of tens of nanoseconds. One example of aQ-switched medical laser is disclosed in U.S. Pat. No. 5,217,455, issuedJun. 8, 1993 to Tan. This patent discloses a Q-switched, tunable solidstate alexandrite laser which generates an output in the 600 to 1100nanometer range. The duration of the q-switched pulses is 10 to 300nanoseconds.

In addition to solid state lasers, tunable dye lasers have also beenused for treatment of pigmented lesions. For example, U.S. Pat. No.5,312,395, issued May 17, 1994, to Tan relates to a dye laser having anoutput of 345 to 600 nanometers. The patent suggests that the durationof the output pulses should be 500 nanoseconds or less.

In order to provide a wider range of treatment options, it has beensuggested that medical laser systems include more than one type oflaser. For example, PCT Application No. WO 91/13652, published Sep. 19,1991, discloses a laser system where both an alexandrite laser and a dyelaser are combined in one housing.

It has been recognized that it would be desirable to generate high powerpulses having a duration longer than is available in prior art medicallaser systems. This problem was addressed in U.S. Pat. No. 5,287,380,issued Feb. 15, 1994, to Hsia. This patent relates to a flash lamppumped dye laser. A flashlamp power circuit is disclosed which ramps upthe amplitude of the drive current in order to increase the pulse lengthabove 500 microseconds. By using the approach in the Hsia patent, anoutput pulse of 640 microseconds was created.

The inventors herein believe that the effectiveness of the treatment canbe further enhanced if the pulse width can be even further lengthened.More specifically, it is believed that when pulses widths on the orderof 500 microseconds or less are used, the laser energy tends to boil theblood in the veins being treated. When the blood is boiled, there israpid expansion, bleeding and immediate purpura (bruises).

In preliminary investigations, the inventors herein have shown thatimproved results can be achieved with pulse widths in excess of 500micro-seconds. When longer pulse widths are used, the veins tend to becoagulated without boiling the blood. As noted above, there is an upperlimit on the ideal pulse width, since longer pulses result in excessthermal damage beyond the treatment site. Therefore, it is believed thanan ideal system would be able to generate output pulses having aduration between 0.5 and 10 milliseconds, at a wavelength which isabsorbed in the blood and having sufficient power to coagulate the vein.

Accordingly, it is an object of the subject invention to develop a lasersystem which can generate a long pulse output with sufficient power tocoagulate and collapse veins.

SUMMARY OF THE INVENTION

This object is achieved in the subject invention wherein a laser systemis provided which is capable of generating pulses exceeding 0.5milliseconds, having a wavelength of 532 nm which is readily absorbed inthe blood and having an energy of at least 0.5 joules per pulse. Thelaser system includes a neodymium doped solid state gain medium having afundamental output wavelength of 1.06 microns. In accordance with thesubject invention, a non-linear crystal, such as KTP, is located in theresonator at a focal point of the circulating beam. The non-linearcrystal functions to double the frequency of the fundamental wavelengthand generate output pulses at 532 nm.

A flashlamp is used to energize the gain medium. In accordance with thesubject invention, the flashlamp pulses are arranged to generate outputpulses in excess of 0.5 milliseconds and preferably between 0.5 to 10milliseconds. The energy per pulse is on the order of 0.5 to 3.0 joules.It has been shown that pulses of this character can be used to coagulateunwanted blood vessels with a minimum of bleeding and pain.

It should be noted that intracavity, frequency doubled Nd:YAG lasershave been used in the prior art to treat vascular lesions. However, tothe applicants knowledge, those systems have been operated with aQ-switch to generate very short pulses having a high peak power. It isbelieved that the subject long pulse, intracavity doubled Nd:YAG is thefirst laser of this type to be used for this purpose.

Further objects and advantages of the subject invention will becomeapparent from the following detailed description taken in conjunctionwith the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic diagram of the laser system of thesubject invention.

FIG. 2 is a top plan view of a preferred form of the laser system of thesubject invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is illustrated a schematic diagram of thelaser 10 of the subject invention. The laser includes a neodymium dopedsolid state rod 20. The host crystal is preferably YAG, but could be anyof the other standard hosts such as YLF and YSGG. When excited by aflashlamp 22, the Nd:YAG rod emits an output wavelength of 1.06 microns.This output is circulated within a resonant cavity bounded by highlyreflective end mirrors 30 and 32.

Also included within the resonant cavity is a non-linear crystal 36.Non-linear crystal is provided to double the frequency of thefundamental wavelength generated by the Nd:YAG crystal 20. Suitablecrystals for converting the 1.06 micron radiation into 532 nm lightinclude KTP, BBO and KDP.

In the preferred embodiment, crystal 36 is located in a focusing branchof the resonator defined between curved end mirror 32 and curved mirror40. It is desirable to focus the light within the crystal to increasethe doubling efficiency.

Mirrors 30, 32 and 40 are provided with a coating which is highlyreflective at both 1.06 microns and 532 nm. An output coupler mirror 44is provided which is highly reflective at 1.06 microns and highlytransmissive at 532 nm. By this arrangement, the doubling effect occursthrough two passes through crystal 36. Light coupled out of theresonator through coupler 44 can be delivered to a vein 46 at thetreatment site through either a fiber optic element or a hollowwaveguide channel.

A control circuit 48 is provided for regulating the power supply 50. Inoperation, the control circuit will signal the power supply to energizethe flashlamp. In the preferred embodiment, the flashlamp drive pulseshave a duration between 0.5 and 10.0 milliseconds. At these pulsewidths, a significant amount of intracavity power can be built up toenhance the efficiency of the doubling process.

FIG. 2 is top plan view of the lay out of the preferred form of thesubject invention. The optical elements of the laser are mounted on ahousing 12. A laser head 14 includes a Nd:YAG rod 20 and the flashlamp22. The flashlamp and rod are water cooled with a circulation system ina conventional manner.

The gain medium 20 lies within the resonator defined by curved mirrors30 and 32. The spacing of the mirrors is set to optimize performance at1.06 microns. The non-linear crystal 36 is located within a focusingbranch of the resonator defined by end mirror 32 and curved fold mirror40. Each of these mirrors includes a coating is which highly reflectiveat both 1.06 microns and 532 nm. A flat output coupler 44 is providedwhich is highly reflective (about 98%) at 1.06 microns and highlytransmissive at 532 nm.

A shutter 54 is provided in the cavity which is selectively positionableinto the path of the laser beam. Upon start-up, the shutter is orientedto block the beam. During the first second or two of flashlampoperation, the gain medium 20 will become heated and any thermal lenseffects will tend to stabilize. Once the thermal gradients in the gainmedium have stabilized, the shutter is moved and the beam is permittedto reach the crystal 36. In this manner, the damage to the crystal fromhot spots created by thermal lensing in the gain medium during warm-upis minimized. The use of the shutter also results in a more stableoutput.

A reflective filter 56 is mounted on the housing 12 to reject any 1.06micron radiation which is transmitted past the output coupler. Thisportion of the beam is captured by a beam dump 58. The output beam isthen directing into a fiber focus assembly 60 which includes anadjustable lens for injecting the laser output into a fiber.

Unlike prior art Q-switched system, which generate very short, high peakpower pulses, the pulses generated by the subject system are longer andhave lower peak power. For this reason, the doubling efficiency is lessthan with Q-switched lasers. In tests with the subject system, it isestimated that the doubling efficiency is on the order of 1 to 2percent. Nonetheless, the subject system has been designed to generatepulses having an energy from 0.5 to 3.0 joules. At the longer pulsewidths available from the subject system, high power pulses can begenerated. For example, a one joule, two millisecond pulse will produce500 watts of peak power.

The subject system has been used experimentally in animal studies. Inthe animal studies, albino rabbits were anesthetized by intramuscularinjection of ketamine. The fur was depilated from the dorsal earsurfaces with Neet. Peripheral ear venules were selected and theirdiameters measured under a dissecting microscope. Marker dots wereplaced on either side of each venule at the site to be exposed to thelaser. These assured accurate laser exposure placement and orientationfor histological sectioning perpendicular to the venule.

In each animal, laser exposures were performed in duplicate for 160 and320 μm vessels. The exposure durations were one, five and tenmilliseconds. The fluences varied between 10 and 20 J/cm². Each exposedvessel was observed immediately and at five and ten minutes forresponses including vasodilation, vasoconstriction, apparent flowchanges, closure and hemorrhage. Two to three hours after exposure, thesites were biopsied and fixed in formalin for routine processing andlight microscopic histology after staining with hematoxylin/eosinstains.

Laser pulses of five to ten milliseconds at fluences between 10 and 15J/cm², caused clinically a vasoconstriction reaction in the targetedvessels. Histologically, the endothelial cells in these vessels weredamaged and polymorphonuclear cells stuck to the interior vessel wall.The red blood cells showed partial or complete agglutination. Thevessels were also surrounded by a fine rim of perivascular collagendenaturation. Polarized microscopy showed that the damaged collagen hadalso lost is birefringence. At 20 J/cm², the vascular injury wassimilar, but there was pronounced epidermal and adjacent collagendamage. This was prevented by cooling of the skin during laser exposureswith a cooling chamber.

Based on the above, it can be seen that the subject laser produces anideal output format for treating various vascular lesions. The longerpulse width tends to significantly reduce purpura while still minimizingthermal damage to surrounding tissue.

Based on the results described above, the following general treatmentparameters can be defined for use in human patients.

PULSE DURATION (Pulsewidth)

The ideal pulse duration for treating most portwine stains (PWS) andsmall telangiectasia is 1-10 milliseconds. This corresponds to thermalrelaxation times of vessels approximately 30-100 micron diameter,typical for PWS lesions. This pulse duration therefore achieves thermalconfinement on the order of the vessels, but less mechanical damage andhemorrhage than for sub-millisecond (e.g., pulsed dye laser) pulses. The1-10 millisecond pulse duration also allows heat flow into the vesselwall during the response time, increasing effectiveness of vessel wallcoagulation.

FLUENCE

The fluence needed for treatment with this laser lies between thosetypically used with the sub-millisecond 585 nm dye lasers (58Joules/cm²) and those typically used with longer duration exposures fromargon, krypton, argon-dye, copper vapor, or KTP lasers (25-40Joules/cm²) . As with dye lasers, the ideal fluence also variesinversely with the exposure spot diameter, for spots less than about 5mm, and with skin melanin content (pigmentation). The ideal fluence istypically 12-20 Joules/cm² for spots of 3 mm, and somewhat higher forspots less than 3 mm, in most Caucasians.

PULSE REPETITION RATE

Exposures are produced contiguously on the skin. For manual placement, arepetition rate of up to about 10 Hz is controllable. For speed ofoperation, a rate of at least 1 Hz is desirable.

OVERLAPPING PULSES

One method of application practical with this laser is overlapping(multiple) pulses to a given skin site. Two modes can be used. When atleast 10 seconds are allowed for bulk cooling between pulses, thermaldamage can remain selective for vessels. If gross coagulation isdesired, however, multiple pulses can be delivered faster, e.g., at 1-10Hz until a grey-white color change indicating gross coagulation is seen.

While the subject invention has been described with reference to apreferred embodiment, various changes and modifications could be madetherein, by one skilled in the art, without varying from the scope andspirit of the subject invention as defined by the appended claims.

We claim:
 1. An apparatus for treating vascular lesions comprising:aresonant cavity; a neodymium doped solid state gain medium locatedwithin the resonant cavity for generating an output having a fundamentalfrequency; a flashlamp for exciting the gain medium; a non-linearcrystal located within the cavity for doubling the fundamental frequencyoutput of the gain medium; and means for energizing the flashlamp withpulses having a duration between 0.5 and 10.0 milliseconds to generatefrequency doubled output pulses having a duration between 0.5milliseconds and 10 milliseconds and having an energy of at least 0.5joules per pulse.
 2. An apparatus as recited in claim 1 furtherincluding optical means for focusing the fundamental frequency output ofthe gain medium at a location within the resonant cavity and wherein thenon-linear crystal is positioned at that location.
 3. An apparatus asrecited in claim 1 wherein said gain medium is Nd:YAG.
 4. An apparatusas recited in claim 1 wherein said non-linear crystal is KTP.
 5. Anapparatus as recited in claim 1 wherein said pulses have a repetitionrate greater than 1 Hz.
 6. An apparatus as recited in claim 1 whereinsaid output pulses have a duration greater than 1.0 milliseconds.
 7. Anapparatus as recited in claim 1 wherein said output pulses have aduration greater than 5.0 milliseconds.